1. Field of the Invention
The present invention relates to a sliding mode control method with a feedforward compensation function capable of achieving a control response characteristic adapted to varying system parameters, and more particularly, to a control method in which a manipulated variable affecting a controlled object can always be kept proper.
2. Description of the Related Art
A conventional control system for controlling the operation of a robot, machine tool, etc. is generally arranged to carry out proportional-plus-integral control using a fixed gain. In this case, if a system parameter greatly varies, the response characteristic of the control system is likely to become maladapted. In this respect, sliding mode control, etc. have recently been proposed. The conventional sliding mode control, however, entails a drawback such that a steady-state deviation from a control target value is likely to occur.
If the conventional sliding mode control is applied to a control system, which is provided with a feedforward compensation function for improvement of the control response characteristic, the manipulated variable (control input to a controlled object) is likely to become maladapted to the operating state of the controlled object, due to a compensation effect of a feedforward term associated with a nonlinear parameter (e.g., inertia, dynamical friction force, or gravitational term of the robot), when the nonlinear system parameter varies. If the manipulated variable is maladaptive, the controlled variable to be adjusted to a target value oscillates around the target value. In other words, an energy loss occurs in the controlled object. For example, the torque command (manipulated variable) supplied to the controlled object including a servomotor as a drive source oscillatorily varies with a change of the nonlinear parameter. Thus, a driving current flowing through the servomotor periodically becomes excessive, thereby heating the motor.
According to the conventional method neglecting a variation of the nonlinear system parameter in determining the manipulated variable as described above, the manipulated variable is likely to become maladaptive, causing inconveniences.